Tidal devices

Tidal energy exploits the natural ebb and flow of coastal tidal waters caused principally by the interaction of the gravitational fields of the earth, moon and sun. The fast sea currents are often magnified by topographical features, such as headlands, inlets and straits, or by the shape of the seabed when water is forced through narrow channels. The tidal stream devices, which utilise these currents, are broadly similar to submerged wind turbines and are used to exploit the kinetic energy in tidal currents. Due to the higher density of water, this means that the blades can be smaller and turn more slowly, but they still deliver a significant amount of power. To increase the flow and power output from the turbine, concentrators (or shrouds) may be used around the blades to streamline and concentrate the flow towards the rotors.

We have identified six main types of Tidal Energy Convertors (TEC):

A) Horizontal axis turbine

Horizontal axis turbines extract energy from moving water in much the same way as wind turbines extract energy from moving air. The tidal stream causes the rotors to rotate around the horizontal axis and generate power.

B) Vertical axis turbine

Vertical axis turbines extract energy from the tides in a similar manner to that above, however the turbine is mounted on a vertical axis. The tidal stream causes the rotors to rotate around the vertical axis and generate power.

C) Oscillating Hydrofoil

A hydrofoil is attached to an oscillating arm. The tidal current flowing either side of a wing results in lift. This motion then drives fluid in a hydraulic system to be converted into electricity.

D) Enclosed Tips (Venturi)

Venturi Effect devices house the device in a duct which concentrates the tidal flow passing through the turbine. The funnel-like collecting device sits submerged in the tidal current. The flow of water can drive a turbine directly or the induced pressure differential in the system can drive an air-turbine.

E) Archimedes Screw

The Archimedes Screw is a helical corkscrew-shaped device (a helical surface surrounding a central cylindrical shaft). The device draws power from the tidal stream as the water moves up/through the spiral turning the turbines.

F) Tidal Kite

A tidal kite is tethered to the sea bed and carries a turbine below the wing. The kite ‘flies’ in the tidal stream, swooping in a figure-of-eight shape to increase the speed of the water flowing through the turbine.

G) Other Designs

This covers those devices with a unique and very different design to the more well-established types of technology or if information on the device’s characteristics could not be determined.

Methods to fix the TEC to the seabed:

In addition to the categories of devices identified above, there is also a range of methods to fix the converter to the seabed:

i) Seabed mounted / gravity base

This is physically attached to the seabed or is fixed by virtue of its massive weight. In some cases there may be additional fixing to the seabed.

ii) Pile mounted

This principle is analogous to that used to mount most large wind turbines, whereby the device is attached to a pole penetrating the ocean floor. Horizontal axis devices will often be able to yaw about this structure. This may also allow the turbine to be raised above the water level for maintenance.

iii) Floating (with three sub-divisions)

Flexible mooring: The device is tethered via a cable/chain to the seabed allowing considerable freedom of movement. This allows a device to swing as the tidal current direction changes with the tide.

Rigid mooring: The device is secured into position using a fixed mooring system, allowing minimal leeway.

Floating structure: This allows several turbines to be mounted to a single platform, which can move in relation to changes in sea level.

iv) Hydrofoil inducing downforce:

This device uses a number of hydrofoils mounted on a frame to induce a downforce from the tidal current flow. Provided that the ratio of surface areas is such that the downforce generated exceeds the overturning moment, then the device will remain in position.